Chet Haase's Blog

Games Archives

Java Game Development: JDJ article and code

At the end of our JavaOne talk on Desktop Game Development this year, I opened my big mouth and asked who wanted to see the code we'd written to our game demo (a simple version of a well-known paddle-and-ball type game, cleverly named "Ping"). Just about the whole room raised their hands.

Dang! And I thought we were finally done with JavaOne content for the year.

We couldn't very well just post the code alone, and our presentation didn't have enough extra material to fill in the gaps. So we decided to write an article around the whole topic; Java game frameworks and issues, 2D rendering, and graphics performance tips.

Well, it took a couple of months, but Dmitri and I have finally written and published the article. It's a two-part series starting in the October issue of Java Developers Journal.. You can also check out the source code for the demo.

Part 1 is all about the game framework and related development issues. Part 2 (next month's issue) will dive into 2D rendering and performance specifics.

Intermediate Images

"Freebird!"

I'm a little afraid of posting this blog, thinking that it could resulting in either

• So many and varied requests that it will be difficult to address them all

• No responses at all. (If a blog falls in the forest and noone reads it, does it still make a sound?)

Nevertheless, I'll forge ahead. After all, the whole point in my blogs/articles is to talk about stuff that developers want or need to know more about.

So, in the words of the unforgettable sourthern rock band and Java desktop client developers Lynyrd Skynyrd:

What is it you want to hear?

In other words, what would you like to see articles on? I have a few ideas kicking around in my head that I'd like to cover, and people have suggested a few more. But if you have other ideas that you would like to have considered, please tell me in the feedback section below.

Here are a few topics that I would like to cover sometime soon, to give you a feel for where things are headed. If you have opinions on these, feel free to post those below as well:

• Managed Images: I've written about Managed Images in various forums so far (such as the BufferedImage (Part 1 and Part 2), VolatileImage (Part 1 and Part 2), and Image Strategies articles). But maybe managed images deserve their own dedicated article.
• Old vs New: I believe that some of the confusion and performance problems in Java graphics usage comes from the difference between the old Java graphics APIs (those written in the jdk 1.x days, e.g. ImageProducer, ImageConsumer, PixelGrabber) and the new ones (starting from jdk 1.2, e.g. BufferedImage). I would like to write an article (or maybe a series of articles) on the differences between these APIs and their implementations. Specifically, I would like to examine how developers would do things (such as image pixel manipulation) on the old graphics APIs and how they would do things using the newer APIs (hopefully better, faster, and more efficiently).
• Server-based graphics: This is also probably a series of articles. One thread is a discussion of how to do graphics operations (such as image creation and manipulation) in server apps (when you are not displaying to screen, but rather to/from images only). Another thread involves rich clients for server-based applications (such as Web Services); how can we make it easier to write clients that interact on the network and with servers? These are all pretty complex topics, so understanding and explaining the issues may take some time. But in this increasingly connected world, this seems like a useful topic in graphics development.
• ImageIO Utilities: While ImageIO simplified many things in image reading/writing and manipulation, it complicated some operations that are simpler in older APIs. For example, displaying an animated GIF is simple when loaded through some of the old 1.x APIs such as Toolkit.getImage(). But when using ImageIO, the developer needs to do much more work to display the images correctly. It's that whole "with great power comes great responsibility" thing; the power of the ImageIO APIs and the information that it exposes to the developer also (for now) necessitates more work to make some operations succeed. I would like to explore some of these issues to see if we can derive some simple frameworks or approaches that developers can reuse in their work to simplify their applications.
• Image Scaling: This is related to the "Old vs. New" topic above; some operations in our APIs are not obvious. Also, even when there are obvious methods in the API for accomplishing tasks, sometimes it is not clear how these methods work, or that the best ways of accomplishing the tasks are perhaps not related to the most obvious means. For example, Image.getScaledInstance() is not, in general, the best way to scale an image, either performance- or quality-wise. The Graphics.drawImage() methods are much better in general, but they are not necessarily obvious to developers new to the Graphics class. I would like to cover some of these areas and discuss the merits of the different approaches.
• Rendering Hints: Wouldn't it be nice to have the hints discussed in a single area where it is more clear (than in the current javadocs) what happens when various hints are enabled?
• Transform Tips & Tricks: There are always questions about how to go about transforming things correctly or most efficiently. We could explore some of these topics in an article or series of articles.
• Animation: Some of the *Image* articles have touched on this topic, but perhaps an article dedicated to animation would be helpful. This article could cover everything from performance tips to timer usage.
  Done! I've just posted an article entitled Timing is Everything on java.net. It doesn't cover animation as a whole, but has some information about timers and utility code I developed to add higher-level functionality to the existing Java timers.
  
• Intermediate Images: This topic comes out of something we've discussed in our JavaOne talks before: using images to cache complex rendering. This can be a huge win for animation or performance-sensitive graphics apps. Some sample code and implementation details here might help people take advantage of this high-performance approach to Java graphics development.
  Done! An article on Intermediate Images is now up on java.sun.com.
  

That's all I can think of for now, although I tend to add to my internal list of blog TODOs frequently.

ToolkitBufferedVolatileManagedImage Strategies

A common question seems to arise often from Java graphics developers about which image type or creation method to use. When exactly should you use VolatileImage? What is BufferedImage appropriate for? What about the old Toolkit images? And when is BufferStrategy more appropriate than one of these image types?

It's a pretty big topic, and the answer (like all truly great answers) is probably "It depends". But there are some general guidelines that can come in handy. And perhaps a description of what these different kinds of images and methods are all about might help.

1) Image Types

First of all, perhaps a short dictionary of image types might help:

• Toolkit Image: This is the oldest kind of image object in the Java API. These images are created and returned by the old 1.x APIs such as Applet.getImage() and Toolkit.createImage(). These images are created from a pointer to a data source, such as a GIF or JPG file, and return an object of type Image. They are useful for convenient loading and storage of image data for display, but getting at the actual pixel data or manipulating it is not as easy.
• BufferedImage: This is an image type that was created in the jdk1.2 API. They were created for easier and more powerful manipulation of the actual pixel data in an image. At first, there was no way to load an image from a data source directly into a BufferedImage; these images were used, instead, to create an arbitrary buffer for pixel data, which you could then write to, read from, or display conveniently. The main way to get actual pixel data into a BufferedImage object at first was through use of rendering operations (after getting a Graphics object for the BufferedImage), or by manually setting the pixel data through methods in BufferedImage, WritableRaster, and DataBuffer. With the advent of the ImageIO API (see below) in jdk1.4, it became possible to create a BufferedImage object directly from a data source, just like Toolkit images (only these BufferedImage objects are writable, unlike their Toolkit image cousins).
• VolatileImage: This image type was created in jdk 1.4 as a means of creating and managing accelerated image memory. One of the problems with hardware acceleration for images is that, on some platforms, accelerated memory can be deleted out from under you at any time. This is obviously not what you want for your typical image data. To work around that, the VolatileImage API was created to provide a notification mechanism so that you know when an image must be re-rendered due to data loss. VolatileImage objects are not loaded from image data, but are just created as empty pixel buffers (much as the initial BufferedImage objects were (see above)); to get loaded image data into a VolatileImage, applications must load the image data through some non-Volatile means, get the Graphics object for the VolatileImage, and then copy the data into the Graphics object using drawImage().
• Managed Images: These image objects are not specific objects or APIs in Java, but are rather a concept of how we accelerate image operations. A "managed image" is one that you create through any of the normal image creation/loading methods and which we try to accelerate for you internally, by creating an accelerated mirror copy of the image data. This type of image can benefit from hardware acceleration without falling prey to the "surface loss" issues mentioned above for VolatileImage. I'll talk more about managed images and hardware acceleration later in the article.

That's it for the basic image types. Now let's talk about how we actually create and use these image objects.

2) Who you gonna call?

Whenever I want to give myself a fright about the complexity of our APIs, I simply ponder the vast array of choices that face developers who simply want to create an image. I'm sure I'm missing some here, but let's see...

Applet:
	getImage(url)
	getImage(url, name)

BufferedImage:
	new BufferedImage(colorModel, raster, premultiplied, properties)
	new BufferedImage(width, height, type)
	new BufferedImage(width, height, type, colorModel)

BufferStrategy:
	new BufferStrategy()
	(Note: This method cannot be called directly)

Canvas:
	createBufferStrategy(numBuffers)
	createBufferStrategy(numBuffers, capabilities)
	
Component:
	createImage(imageProducer)
	createImage(width, height)
	createVolatileImage(width, height)
	createVolatileImage(width, height, capabilities)

GraphicsConfiguration:
	createCompatibleImage(width, height)
	createCompatibleImage(width, height, transparency)
	createCompatibleVolatileImage(width, height)
	createCompatibleVolatileImage(width, height, capabilities)

Image:
	new Image()
	(Note: This method cannot be called directly)

ImageIO:
	read(file)
	read(imageInputStream)
	read(inputStream)
	read(url)

ImageIcon:
	new ImageIcon(imageData[]).getImage()
	new ImageIcon(imageData[], description).getImage()
	new ImageIcon(image).getImage()
	new ImageIcon(image, description).getImage()
	new ImageIcon(filename).getImage()
	new ImageIcon(filename, description).getImage()
	new ImageIcon(url).getImage()
	new ImageIcon(url, description).getImage()

Toolkit:
	createImage(imagedata[])
	createImage(imagedata[], offset, length)
	createImage(producer)
	createImage(filename)
	createImage(url)
	getImage(filename)
	getImage(url)

Window:
	createBufferStrategy(numBuffers)
	createBufferStrategy(numBuffers, capabilities)

I'm sure there's more out there, especially using things like ImageIO (which is all about reading and writing images, as you might guess from the name...). But this list will do for now.

So it's a wrap. This article's pretty much finished; just use the above API calls to create your images. Left as an exercise to the reader. Q.E.D. It's obvious, isn't it?

Okay, so maybe it isn't obvious; there are a lot of methods above that all seem to need different parameters or that create different types of images.

Here's the trick: All of the above image creation methods (and any others that are not on the list) can be broken down into just a few categories. Then the plethora of ways of creating an image in one of those categories can just be seen as utility methods; different ways of getting the same result. The convenience methods may be because of logic (why do I have to get the GraphicsConfig to create an image associated with a Component? Why not use the Component directly?), or convenience (instead of using some InputStream mechanism for all image readers, we provide several ways to read the image directly including from filenames, URLs, and streams; just call the method appropriate for your situation).

So the real work in this article is to break down the categories of image types and describe which types of images and methods you may want to use in which situations. Once you get that down, the rest, as they say, is just implementation details.

3) Image Loading or Creation?

First of all, are you loading existing image data? Or are you creating an image buffer in memory? Image loading means that you have image data (either locally or across the network) that you want to load into your application, possibly to copy that image onto the screen or to read and operate on the data. Image creation means that you want some arbitrary image memory created for your application; perhaps you want to create a buffer for double-buffered animations, or you want a place to cache intermediate filtering results.

3.1) Image Loading

In the above method list, all of the methods that take filenames, urls, streams, producers, and data arrays are those intended for loading existing images. In particular, all of the methods listed above for Applet, ImageIO, ImageIcon, and Toolkit are intended for image loading:

Applet:
	getImage(url)
	getImage(url, name)

ImageIO:
	read(file)
	read(imageInputStream)
	read(inputStream)
	read(url)

ImageIcon:
	new ImageIcon(imageData[]).getImage()
	new ImageIcon(imageData[], description).getImage()
	new ImageIcon(filename).getImage()
	new ImageIcon(filename, description).getImage()
	new ImageIcon(url).getImage()
	new ImageIcon(url, description).getImage()

Toolkit:
	createImage(imageData[])
	createImage(imageData[], offset, length)
	createImage(producer)
	createImage(filename)
	createImage(url)
	getImage(filename)
	getImage(url)

There are at least four major things that differentiate these methods:

• the location of the image data
• the format of the image data
• the synchronous or asynchronous behavior of each method
• the type of image that is created (Image or BufferedImage)

When I'm talking about location, I'm mainly concerned with whether the file is local or across a network. Also, if it's packed into some resource file, such as a jar file, that also comes into play here.

Loading across the network

If you are accessing the data across a network, it's probably easiest to use the URL variations:

	Applet.getImage(url)
	Applet.getImage(url, name)
	ImageIO.read(url)
	new ImageIcon(url).getImage()
	new ImageIcon(url, description).getImage()
	Toolkit.createImage(url)
	Toolkit.getImage(url)

	URL dukeURL = new URL("http://java.sun.com/duke.gif");
	BufferedImage img = ImageIO.read(dukeURL);

Now suppose you have another image that you have saved locally in a file; just use the filename variation of the above. For example, let's say you loved one particular instantiation of the lovable-yet-quirky duke.gif file above so much that you downloaded and saved it for use in your application (see the above note on scary lawyers). Then you could use the following code to load that file from the directory where the program was launched:

	BufferedImage img = ImageIO.read(new File("duke.gif"));

	URL dukeURL = getClass().getResource("media/" + "duke.gif");
	BufferedImage img = ImageIO.read(dukeURL);

Another consideration is the format of your stored image. The old Toolkit/Applet loaders only understand GIF, JPEG, and PNG format files. (Okay, they also understand XBM and XPM2, old X11 image formats, but those are probably not formats you are terribly concerned about). These loaders works well for most web applications since these image types are traditional web image formats. But what if you have an image in some other format that the Toolkit/Applet loaders do not understand?

ImageIO currently has built-in readers for GIF, JEG, and PNG. In addition, it will have BMP and WBMP capability in the jdk1.5 release. Moreover, there will be more image readers/writers for ImageIO going forward, whereas there are no specific plans to support more formats for the old Toolkit/Applet loaders. And finally, ImageIO has a pluggable reader API, so if you have a custom image format, or some other format not yet supported by the core library, you can write your own loader for that format within ImageIO.

In fact, the JAI team has made available a package with additional ImageIO readers/writers at http://java.sun.com/products/java-media/jai/downloads/download-iio.html if you have requirements beyond the current ImageIO defaults.

So ImageIO could also be the right choice if you need to deal with formats beyond the basic web image formats.

3.1.3) Synchronicity

The Applet and Toolkit image loading methods came from the old days of Java 1.0, when Java was seen primarily as a networked application API and image data might come from any source, potentially one on an unreliable or slow network connection. To make networked applications more robust, it is reasonable to put network-dependent operations in separate threads to ensure that an application's main or GUI threads do not hang while waiting for a slow download. Because this was a common pattern for Java GUI applications at that time, the image loading operations were all created to run on a separate image loading thread. Thus when an application calls:

	Applet.getImage(url)

Note that this model of asynchronous loading does not apply solely to networked applications, or even to image loading specifically; any operation that takes a significant amount of time should not be done on the GUI thread, lest you run the chance of making your application appear hung while the operation is taking place. So, for example, if you are loading in a huge image from a local file, you may want that non-networked operation to happen in a separate worker thread to ensure that your GUI has no pauses during image loading.

This model works well enough for applications that create their images early for later use. The application simply may need to check whether the image has been loaded whenever it is required in the application.

When applications do need the data (for example, if they need image sizes in order to determine layout correctly, or if they need to display images in their final form), they may need to synchronize on the image loader and wait until the image loading is done. For example, an application may want to load local image data and be willing to wait for that data to load before proceeding (knowing that a local load will usually not take very long). In that case, the application might do something similar to the following:

	public Image loadImageSynchronously(Component comp, String filename) {
	    URL url = getClass().getResource("media/" + filename);
	    Image image = Toolkit.getDefaultToolkit().getImage(url);
	    MediaTracker tracker = new MediaTracker(comp);
	    int id = 0;
	    tracker.addImage(image, id);
	    try {
		tracker.waitForID(id);
	    } catch (InterruptedException e) {
		System.out.println("INTERRUPTED while loading Image");
	    }
	    return image;
	}

	public Image loadImageSynchronously(String filename) {
	    URL url = getClass().getResource("media/" + filename);
	    Image image = new ImageIcon(url).getImage();
	    return image;
	}

Meanwhile, ImageIO has synchronous loading methods that do not return until the image has been loaded and is ready to go. Note that some applications and situations may still need asynchronous loading behavior (for long image loads or to more efficiently multitask). For example, it does not take a huge amount of time to affect perceived GUI performance, so if an image load will take even as long as a tenth of a second, you may want to avoid loading that image synchronously on the Event Dispatch Thread (so don't load it in your paint() method). You can always spawn a new Thread yourself to call the ImageIO loading methods if necessary.

3.1.4) Resulting Java Image Type

Part of the decision over which image creation API you is in which image type you want to get back from the creation method. In particular, do you want a Toolkit Image or a BufferedImage?

Toolkit Images are created by the Applet, Toolkit, and ImageIcon methods listed above. The resulting images are easy to use for display purposes (just call drawImage(...) from this image into a Graphics destination and the image will be copied appropriately), but lack the power of BufferedImage objects for manipulation of the data.

BufferedImage objects are created by the ImageIO methods listed above. These objects offer a more powerful API, albeit with potentially more work involved to do some operations (such as displaying an animating GIF image).

Image or BufferedImage: What's in a Name?

Although both Image and BufferedImage have similar properties in terms of being displayable, BufferedImage has many more capabilities. For one thing, the Image objects created by the Toolkit, Applet, and ImageIcon load methods are read-only; you cannot get the Graphics of those Images and render to them. So if you want to modify the image data, you will need to do more work (such as creating another image that is modifiable and copying the loaded Image into that new image). Image has some very simple methods and is mostly intended to be a simple object that holds image data. But BufferedImage has many methods for modifying and extracting all kinds of data from an image; color models, pixel data, and more. Given a choice between the two, I would always opt for the one that gave me more power and flexibility.

But doesn't that increased capability mean increased overhead? Not at all; there is no extra processing involved in BufferedImages when these other powerful methods are not used. If all you do is load and image and display it, BufferedImage can do this just as easily as the more streamlined Image object.

So go ahead and use BufferedImage. It is, after all, better than butter.

Dirty Laundry

One good (and not entirely obvious) reason for using the ImageIO API for loading images is the unfortunate reality that the code is simply newer, cleaner, and more maintained (both now and in the future). Much of the old Applet and Toolkit image code was written years ago and has many assumptions and situations that it must account for and is therefore tricky to maintain and upgrade.

Our future image reading/writing direction is with ImageIO; yours should be too, because that's where the focus of our efforts will be in the future.

Having said all that wonderful stuff about ImageIO, there could be situations in which the old Toolkit/Applet/ImageIcon approach makes more sense for your particular application, including:

• Performance: Ideally, ImageIO would be more performant than the old code. But some code paths in ImageIO have not yet been optimized to the extent that the old code was, so you may find some situations (such as loading images in a particular format) which perform better using the old APIs than they do with ImageIO. The problems are (mostly) known, the bugs have (mostly) been filed, and these performance gaps will (definitely) be fixed. But if performance in these situations is important for your application, by all means use the old methods in the meantime if they work better for you.
• Ease of Use: One of the goals of ImageIO was to expose more capabilities to the developer. There is much information, such as metadata, in an image that is not exposed through the old image loading methods or the old Image class. The flipside of this approach (at least in the current API) is that it can be more work to do trivial tasks. A good example of this is loading and displaying an animated GIF.
  In the old image loading APIs, if you loaded an animated GIF file, you did not need to worry about the details of animating that image. Instead, you could just load the image and display it and we would automatically schedule the animations of that image appropriately. You could not access the information about the animation (how many frames? what animation rate? loop? halt?), but making it animate was quite simple.
  ImageIO, on the other hand, exposes all of that animation information to you through the API, but you must actually use that information to manage the image animation yourself; you cannot just tell us to draw the image and have us manage it for you. This is a known issue and we will do something about it Real Soon Now; look for an upcoming article about doing this, and look for future API changes to make this simpler in the future.
  
• Compatibility: Sometimes applications need to be as backward-compatible as possible across most or all jdk releases. In particular, some applications need to be able to run on old jdk1.1 or earlier releases. For example, applet games tend to need ubiquity of the runtime environment and cannot count on browsers having any later version of Java than the 1.1 APIs shipped with the MSVM. In this case, you may need to use the older APIs simply so that they will run across all of these platforms.

Note also that if you need to use the old APIs for some reason but you still want the power and flexibility of BufferedImage, it is easy enough to load the images in through whatever methods are appropriate, create a new BufferedImage object, and then simply copy the loaded images into the BufferedImage. For example:

	Image img = new ImageIcon(filename).getImage();
	BufferedImage bImg = new BufferedImage(img.getWidth(null),
					       img.getHeight(null).
					       BufferedImage.TYPE_INT_RGB);
	Graphics g = bImg.getGraphics();
	g.drawImage(img, 0, 0, null);

3.1.5) Hey! What about the other loading methods above?

The approaches above cover most of the loading methods I listed, but some are notably skipped. The *Stream methods of ImageIO are simply variations on a theme; if you happen to have your data in that format (versus a URL or file), go for it; it's just a convenience to use these alternatives.

As for the other skipped methods (one using an ImageProducer and some using data arrays), I hoped you wouldn't notice....
These other loading approaches are somewhat dated and come from the old days of Java image processing. There are still some situations which might require the ImageProducer/ImageConsumer approach, but in general it should be easier, more straightforward, and more performant to use the newer APIs such as ImageIO and BufferedImage. The uses and misuses of the older Consumer/Producer/PixelGrabber APIs could stand a separate article all by themselves. I'll try to tackle this one in the future.

As far as reading the image data from an array of data (see the methods above with the imageData[] parameter), this is really only appropriate if you've already read the data into the array to begin with. This could be necessary if you have some custom image storage mechanism, such as a database. But if the image existing in a regular file/URL/stream format, you should probably be using one of the other loading methods instead.

3.2) Image Creation

What if you do not have an existing image on the network or file system? What if you just want a buffer of pixel data that you can use in your application? This could be for creating sprites or icons with rendering calls instead of loaded image information (perhaps you've found this to be faster in your situation than reading image files). Or it could be a buffer that you can use for caching intermediate results or for providing double-buffered rendering for an animation.

For the purposes of this discussion, I'll break down this category of images into three types:

• Static Images: These are images that you render to infrequently (perhaps just once, when you create the image). The images are mainly used to copy from.
• Dynamic Images: These are images that you render to often, like an animating sprite.
• Back Buffers: These are like dynamic images, in that they are rendered to frequently (usually at least once per frame), but they are specifically intended to provide buffering support for the onscreen window, and are thus usually copied from often as well, like once per frame.

3.2.1) Static Images

Static images are ones that are created and rendered to once (or infrequently) but probably copied from often. Examples of this type of image include icons for a GUI or sprites for a game.

The best approach for this type of image is to create an image that is in the same format as the image or window that the image will be copied to; this ensures the most straightforward copy mechanism since the underlying software will not have to perform a conversion on the image data while copying to the destination.

You could, of course, create a BufferedImage object manually through one of its constructors; you could query the GraphicsDevice for its display information and then create a BufferedImage of the appropriate type:

	new BufferedImage(colorModel, raster, premultiplied, properties)
	new BufferedImage(width, height, type)
	new BufferedImage(width, height, type, colorModel)

But why go to the hassle of all of that when there are convenience mechanisms that do all of this for you? Specifically, take a look at:

	Component.createImage(width, height)
	GraphicsConfiguration.createCompatibleImage(width, height)
	GraphicsConfiguration.createCompatibleImage(width, height, transparency)

The best part about static images is that you can use very simple means to create the images and then we will try very hard internally to see that you get any available hardware acceleration for these images when they get copied around. We call these "managed images", because we manage the acceleration details for you. For more information on managed images, please see my blog on BufferedImage performance.

Note that we currently (in all jdk1.4.* releases) manage images that are created with the above APIs and some of the Toolkit image loading methods described previously, but in jdk 5.0 (available in Beta form now, and full release soon) we manage nearly all types of images and take advantage of hardware acceleration if it exists. So go ahead and create the type of image that is most convenient for you and we'll try to do the right thing under the hood.

3.2.2) Dynamic Images

This kind of image may be rendered to quite often, as in an animating icon, or a sprite that is modified on a frequent basis. You could certainly use the same image-creation APIs listed above for static images; these will work fine in most situations and are certainly the easiest way to go in general.

However, some developers interested in maximizing performance may want to know more about image management and how dynamic images can affect it.

We manage images by detecting when the application is copying from an image to a destination (either another image or an onscreen window) that lives in accelerated memory. If this copy is done successively when the source image has not changed, then we may decide to cache a copy of that image in accelerated memory and perform future copies from this cached version.

In the case of a dynamic image, if that image is being updated one or more times for every copy to the destination, then we will never create an accelerated version of it, and thus the image will never benefit from any hardware acceleration that we could otherwise provide.

(Aside: For the insatiably curious, the reason for this oddity in acceleration comes from "surface loss", where an accelerated version of an image may simple go away at any time due to operating-system or user-caused situations. To keep the original image data intact, we store the main image data (that which is modified by the application) in an unaccelerated location, and only accelerate a mirror copy of that image. That way, if the accelerated version gets wiped out, we still have the original data from which we can create a new accelerated copy. The problem here, in terms of performance, is that an "unaccelerated image" means that all rendering to and from that image is unaccelerated. And if an application is constantly modifying the image, all of that rendering will be unaccelerated and it is never appropriate for us to create and use an accelerated version of that image.)

Developers that care about top performance for these types of images may want to look into using VolatileImages instead. These images store the data in accelerated memory (when possible) and thus rendering to and from that image may be accelerated automatically. The downside is that these images require a bit more care and feeding, due to "surface loss" issues that arise with current video memory architectures and operating systems. Note that not all types of rendering to these images is accelerated, either, but simple types of rendering like lines and rectangular fills and copies can usually be accelerated, depending on the platform configuration.

I've already written about VolatileImages in past blogs (Part I and Part II), so I will not go into the details of their usage here; please check out those other articles for more information. But it is worth covering the APIs used to create the images, just for consistency's sake in this article:

	Component.createVolatileImage(width, height)
	Component.createVolatileImage(width, height, capabilities)
	GraphicsConfiguration.createCompatibleVolatileImage(width, height)
	GraphicsConfiguration.createCompatibleVolatileImage(width, height, capabilities)

	GraphicsConfiguration.createCompatibleVolatileImage(width, height, transparency)
	GraphicsConfiguration.createCompatibleVolatileImage(width, height, capabilities, transparency)

The use of the ImageCapabilities object in these methods gives you the ability to require certain attributes (such as hardware acceleration) from any image created with that method. In general, you probably will not need to use that variation, although as we enable more hardware acceleration features in our platform, we may expand the ImageCapabilities API to be more powerful and useful. (Note, too, that ImageCapabilities can be used effectively as a means of inquiring what capabilities an existing image has).

3.2.3) Back Buffers

By "back buffer" I mean an arbitrary offscreen image that is created for use in a double-buffering situation. Typically, an application that wishes to have smooth graphics, especially animations, will draw to a back buffer and then copy that buffer onto the screen instead of drawing directly to the screen. Swing does this by default, so that you do not see the various GUI elements in a Swing app flash as they are drawn to the screen. The buffer copy in these applications typically happens so fast that the graphics in the application are perceptibly smoother than if they were drawn one-by-one directly to the screen.

A developer could use any of the above static or dynamic image APIs that I listed for creating a back buffer. However, the following things should be taken into account when doing so:

• Use of managed images for buffers has the same performance implications as dynamic images; we will be unable to manage the images effectively, thus you will not be able to take advantage of any hardware acceleration either for rendering to or copying from the buffer. Since the copy from the back buffer to the front buffer involves a lot of pixels, being able to accelerate that operation can have a huge performance impact on your application.
• Use of VolatileImages for buffers is a better way to go in terms of hardware acceleration potential. But why go to all the bother of managing your VolatileImages when there is an easier route?

BufferStrategy: the preferred way of buffering in Java

In jdk1.4, we introduced the BufferStrategy API, which is a wrapper around VolatileImages. This API allows you to ask for an accelerated back buffer and avoid having to manage the details of surface loss associated with VolatileImages. It also ensures that you will get a buffer of the optimal type for your application. In particular, you will get either a FlipBuffer (which can only be used in fullscreen-exclusive mode on Windows) or a BltBuffer (which is used by default for windowed applications). A FlipBuffer performs a swap of the front and back buffers in video memory when you request BufferStrategy.show(). A BltBuffer copies the contents from the back buffer to the front (just as you would if you called drawImage() from a VolatileImage back buffer to the front buffer).

With this API, there is little need to create and manage VolatileImages directly; just let us manage the details for you inside the BufferStrategy implementation.

For more information on BufferStrategy, check out the javadocs; they're pretty clear on how the system works.

The APIs you will need when creating a BufferStrategy are:

	Canvas.createBufferStrategy(numBuffers)
	Canvas.createBufferStrategy(numBuffers, capabilities)
	Frame.createBufferStrategy(numBuffers)
	Frame.createBufferStrategy(numBuffers, capabilities)

4) Wrap-Up

So that's pretty much it. You have image loading methods and image (or buffer) creation methods. And in each category, you have various flavors depending on the location and type of the data, and the type of image you want returned to you. So even though there are a lot of methods listed at the top of this article, they all break down into just a few comprehensible categories and can be used effectively, once you understand the implications of each variation.

Although there is certainly more complexity here than we can cover with a simple table, it might help to break down some of the basic attributes of the image types we have talked about and the reasons to consider one type over another when writing your application:

• Toolkit Image:
  • Creation methods: Applet.getImage(...), Toolkit.createImage(...), Toolkit.getImage(...), ImageIcon.getImage()
  • Most useful for: Simple loading and display of image data, applications that need to use 1.x APIs, asynchronous usage models (except for ImageIcon), loading performance (versus some ImageIO loading operations in the current releases).
  • Less useful for: Image data manipulation, reading/writing individual pixel data within the image, synchronous loading operations (except for ImageIcon), more varied or custom source image formats..
• BufferedImage:
  • Creation methods: new BufferedImage(...), Component.createImage(...), GraphicsConfiguration.createCompatibleImage(...), ImageIO.read(...)
  • Most useful for: Image data manipulation, read/write access to individual pixel data, synchronous loading or creation, various image data formats (both source data that ImageIO can read and Java format (i.e., TYPE_INT_RGB, TYPE_INT_RGBA, etc.).
  • Less useful for: Applications that require jdk 1.x compatibility, easy display of animating GIF images.
• VolatileImage:
  • Creation methods: Component.createVolatileImage(...), GraphicsConfiguration.createVolatileImage(...)
  • Most useful for: Hardware acceleration of dynamic images, more fine-grained control over acceleration properties.
  • Less useful for: Simple image creation and display (applications must use the VolatileImage API to handle surface loss issues)
• BufferStrategy:
  • Creation methods: Canvas.createBufferStrategy(), Window.createBufferStrategy()
  • Most useful for: Easy creation and management of window buffering schemes
  • Less useful for: Access to actual image data of buffer

4.1) Hey! You forgot some methods!

There are still a couple of the creation methods up top that I have not covered yet:

BufferStrategy:
	new BufferStrategy()
Image:
	new Image()

4.1) What About Performance?

It is difficult or impossible for me to write a long block of text or code without thinking about performance. And since some of the users of the APIs above, and image operations in general, care a great deal about performance, I should spend a few words discussing some performance issues to be aware of.

Again, check out my blogs on BufferedImage ( Part I and Part II); I go into much more detail on image management there. Some important things to keep in mind with respect to managed images (and making sure they are benefiting from available acceleration):

• If you want copies from an image to be accelerated but you will be modifying the image frequently, think about VolatileImage instead (see the above discussion on Dynamic Images).
• Careful with that Raster, Eugene! This is a hidden gotcha in the current implementation. Part of the BufferedImage API is a method that allows you to get the Raster of the image (getRaster()). From that Raster, you can create a writable child raster (WritableRaster.createWritableChild()) or get the internal data array for the raster (Raster.getDataBuffer()). Both of these methods end up giving you a copy of the data that we have no control over. Specifically, we cannot tell when you have updated the contents of the image, so we cannot tell whether our cached/accelerated version of the image needs to be updated. So, when you call these Raster methods, we throw up our hands and give up on acceleration entirely.
  If you need to access the pixel data but you still want us to accelerate image copies, call methods which manipulate the data for you without giving you a reference to the data; e.g., BufferedImage.setRGB() and WritableRaster.setPixels() for setting arbitrary pixel values, or any of the Graphics.draw*() or Graphics.fill* methods for setting blocks of pixel values.
• If you are transforming the image while copying it around (say, scaling it to the destination, or rotating it), we will not be able to do that using hardware acceleration with the current implementation. If you anticipate using a particular transformation of an image (like a zoomed-in view or several rotations of a sprite) multiple times, then it might be worth creating a temporary version of that transformed image and letting us manage (accelerate) that transformed version for you.

4.2) What about 5.0?

Most of the APIs I discussed above are pre-5.0, so you can use everything above (except where noted) in the current releases available for download. If you are looking forward to using 5.0 (available in beta form today, in full release Real Soon Now), then I'll mention a couple of tweaks to the above:

• VolatileImage APIs: It will be possible in 1.5 to create a VolatileImage with a transparency parameter. Developers have been asking for that capability for some time; the original createVolatileImage() methods only allowed creation of opaque VolatileImages.
  That's the good news: the API is there. The bad news is that there is no hardware acceleration for these non-opaque images by default because our mechanisms for accelerating these types of images are still not ready for prime time. Translucent volatile images can achieve hardware acceleration by using the handy -Dsun.java2d.translaccel=true flag.
• Managed images everywhere! You will now get managed images by calling most of our image creation and loading mechanisms. In jdk1.4.*, only some of the image creation methods (such as Component.createImage() and GraphicsConfiguration.createCompatibleImage()) returned managed images. But in 1.5, you can even call new BufferedImage(...) with a type that is not in the screen bit depth and end up with an accelerated managed image.
• OpenGL Acceleration: The 5.0 release has an OpenGL-based rendering pipeline available on all platforms. This pipeline is not be enabled by default (due to various concerns including graphics hardware and driver issues), but there is a flag that you can use to experiment with it (sun.java2d.opengl=true). This pipeline provides our most comprehensive hardware acceleration so far on all our supported platforms, so it should be pretty exciting to see this one in action.

ImageIO: Just another example of better living by doing it yourself

I've always known that ImageIO was a good thing to use since its inception. It reads and writes more formats than the original Image loading APIs, it has a pluggable interface for new image formats, it's the way of the future, it's more robust than the previous APIs, it's synchronous without the need for MediaTracker or that hacky ImageIcon workaround, blah, blah, blah....

I knew all that without actually working with it because it was apparent from the API. It is the way of the future for image reading and writing, and it is more robust and all the rest of that stuff. It's just a great full-featured package that's written from the ground up to provide what developers have been asking us to provide after using the old Image APIs for years.

But I'd never actually used the darned package...

(Petty justification: The folks working on the libraries for Java don't usually get enough time to play around with using the APIs; we're too busy making the stuff work. That's not to say that the people who wrote ImageIO didn't use it substantially in implementing it; of course they did. But I wasn't involved in that part of the API, and I'm busy enough in my little rendering and performance closet that I didn't take the opportunity to play around with ImageIO since it first came out.)

This all changed in the last month.

See, JavaOne was coming, which meant that I had a few demos to write. And it turns out I needed a couple of image utilities, and I needed them quickly. For both utilities, I began thinking "Now where could I find some simple program to do this ..." and then I realized how easy it would be to accomplish it from scratch in ImageIO. A few lines of code later and, presto!, I had my applications.

Not only did I get exactly the functionality that I was looking for, but I got the satisfaction of actually having written the code myself, which is so darned gratifying to us geeks. As they say, "Just another example of better living by doing it yourself." Or at least that's what I say.

Anyway, on with the code.

ImageScaler

The first utility came from a need to scale some images. We had a demo that was going to show images in a particular size all the time. Rather than scaling them on the fly or caching the scaled versions in the application, we figured it would be better to simply load images that were already of the size needed.

Knowing how these projects tend to go, what I really wanted was the ability to scale our full-size images (always keep the original full-size versions of media around...) down to some arbitrary size. And then, when the designers changed their minds about the size at the last minute, be able to quickly re-scale the images to some other arbitrary size.

We could do this through some image manipulation program, but it would be a major hassle to keep doing it (including having to go through the person that knew the program that ran the macro that ... you get the picture). So what I wanted was a simple standalone application to scale the images.

What I wrote was ImageScaler. This application takes the pathname to a directory (or uses "." as the default) and width/height values. It reads in all image files from that directory, creates a subdirectory ("scaled/"), and writes out scaled versions of those images in the specified width and height.

Here's the code:

package jcg.lhdemo;

import java.awt.*;
import java.awt.image.*;
import javax.imageio.ImageIO;
import java.io.File;

/**
 * ImageScaler
 *
 * This class loads all images in a given directory and scales them to the
 * given sizes, saving the results as JPEG files in a new "scaled/" subdirectory
 * of the original directory.
 */
public class ImageScaler {
    
    // Default w/h values; overriden by command-line -width/-height parameters
    static int IMAGE_W = 150;
    static int IMAGE_H = 250;
    
    public static void main(String args[]) {
        // Default directory is current directory, overridden by -dir parameter
        String imagesDir = ".";
        for (int i = 0; i < args.length; ++i) {
            if (args[i].equals("-dir") && ((i + 1) < args.length)) {
                imagesDir = args[++i];
            } else if (args[i].equals("-width") && ((i + 1) < args.length)) {
                IMAGE_W = Integer.parseInt(args[++i]);
            } else if (args[i].equals("-height") && ((i + 1) < args.length)) {
                IMAGE_H = Integer.parseInt(args[++i]);
            }
        }
        // new subdirectory for scaled images
        String scaledImagesDir = imagesDir + File.separator + "scaled";
        // directory that holds original images
        File cwd = new File(imagesDir);
        // directory for scaled images
        File subdir = new File(scaledImagesDir);
        subdir.mkdir();
        File files[] = cwd.listFiles();
        // temporary image for every scaled instance
        BufferedImage scaledImg = new BufferedImage(IMAGE_W, IMAGE_H, 
                                                    BufferedImage.TYPE_INT_RGB);
        Graphics2D gScaledImg = scaledImg.createGraphics();
        // Note the use of BILNEAR filtering to enable smooth scaling
        gScaledImg.setRenderingHint(RenderingHints.KEY_INTERPOLATION,
                                    RenderingHints.VALUE_INTERPOLATION_BILINEAR);
        for (int i = 0; i < files.length; ++i) {
            try {
                // For every file in the directory, assume it's an image and
                // load it
                BufferedImage img = ImageIO.read(files[i]);
                // If we get here, we must have read the image file successfully.
                // Create a new File in the scaled subdirectory
                File scaledImgFile = new File(scaledImagesDir + File.separator + 
                                              files[i].getName());
                // Scale the original image into the temporary image
                gScaledImg.drawImage(img, 0, 0, IMAGE_W, IMAGE_H, null);
                // Save the scaled version out to the file
                ImageIO.write(scaledImg, "jpeg", scaledImgFile);
            } catch (Exception e) {
                System.out.println("Problem with " + files[i]);
            }
        }
    }
}

• It assumes that the images are in JPEG format. Actually, it does read in the files in any format, but it writes them out in JPEG format with the original filenames, so if the images weren't JPEG to begin with, things will get a bit confusing. We could, of course, use any format that ImageIO supports (or any for which we have plugins); in this case I knew that we were dealing with JPEG images so I made that simplifying assumption in the code. The code could be easily extended to handle other or multiple formats instead.
• Note the use of the BILINEAR rendering hint to the Graphics2D object; this tells Java 2D to do some smooth filtering on the image to make the scaled version look better than it would by using the default NEAREST_NEIGHBOR method. NEAREST_NEIGHBOR works great for many purposes (and is faster in general), but if you're trying to get higher quality and don't mind waiting just a tad longer for the result, use BILINEAR. And for those developers using jdk 5.0, note that the BICUBIC hint now works; you may get even better results with that value.
• Aspect ratio: This code assumes that the original images are in the same aspect ratio as that determined by the width/height that we are scaling to. This may not be the case in any given situation, and might cause unattractive artifacts with widely differing aspect ratios. A more robust application could provide the ability to preserve the aspect ratio of the original instead of using the width/height parameters blindly.
• A batch processing application like ImageScaler which acts on several files in the same format in a loop might be more performant by locating the appropriate reader and writer once, outside the for() loop, and then just calling the appropriate read() or write() functions inside the loop. I'll leave this as an exercise for the reader....

JpegConverter

A couple of weeks after I wrote and used ImageScaler, I ran into another problem where I needed a quick conversion utility. I had a series of BMP images that I wanted to convert to JPEG (in this case, because I needed a compressed image format). Once again I found myself writing a very quick program using ImageIO that did the job. In fact, I probably wrote and ran the program in less time than it would have taken me to find an appropriate utility out on the net, install it, and run it. And, once again, "it's another example of better living by doing it yourself".

This program takes a pointer to a directory (or uses "." by default), reads in all BMP image files in that directory (I constrained JpegConverter to convert only BMP files, but, like ImageScaler, this application could be easily extended to handle multiple formats) and then saves out a JPEG version of each file.

Here's the code:

import java.awt.*;
import java.awt.image.*;
import javax.imageio.ImageIO;
import java.io.File;

/**
 * JpegConverter
 *
 * This class loads all BMP images in a given directory and saves each as a JPEG
 * file in the same directory.  This code is specific to BMP, but it could be
 * easily extended to read images of any type that ImageIO handles.
 */
public class JpegConverter {
    
    public static void main(String args[]) {
        // Default directory is current directory, overridden by -dir parameter
        String imagesDir = ".";
        for (int i = 0; i < args.length; ++i) {
            if (args[i].equals("-dir") && ((i + 1) < args.length)) {
                imagesDir = args[++i];
            }
        }
        // directory that holds original images
        File cwd = new File(imagesDir);
        File files[] = cwd.listFiles();
        for (int i = 0; i < files.length; ++i) {
            String fileName = files[i].getName();
            // converstion to lower case just for ease of replacing the
            // filename extension
            String fileNameLC = fileName.toLowerCase();
            if (fileName.endsWith("bmp")) {
                try {
                    // Replace original "bmp" filename extension with "jpg"
                    int extensionIndex = fileNameLC.lastIndexOf("bmp");
                    String fileNameBase = fileName.substring(0, extensionIndex);
                    BufferedImage img = ImageIO.read(files[i]);
                    // create new JPEG file
                    File convertedImgFile = 
                            new File(imagesDir + File.separator + 
                                     fileNameBase + "jpg");
                    // store original file out in JPEG format
                    ImageIO.write(img, "jpeg", convertedImgFile);
                } catch (Exception e) {
                    System.out.println("Problem with " + files[i]);
                }
            }
        }
    }
}

Notes:

• Some of the notes for ImageScaler above apply to JpegConverter as well, so consider this note as a dereference to the notes above...

And so, in conclusion

Some readers might wonder: "What's the big deal?" After all, these applications are pretty simple, so why all the fuss?

Exactly my point; thanks for clarifying it. It is easy. So easy that I would encourage everyone to think about using ImageIO next time you have a simple (or hard) image input/output problem come up; see if the ImageIO API doesn't make solving that problem just a tad easier.

VolatileImage Q&A

I originally wrote this as one single blog, but as I found out with my previous BufferedImage articles, I tend to go on much longer than I inteded at first, so I had to split it up. So now there's 2 parts to this blog: last time's VolatileImage usage discussion, and this week's VolatileImage FAQ. This week, I thought I'd write down various reasonable questions about either VolatileImage objects and usage in general, or questions that might arise from last week's example. These are questions that come up all the time, or that might reasonably surface from the discussion so far. If you have questions of your own, feel free to post them in the talk-back section at the end.

First, let's see that sample code again:

import java.awt.Color;
import java.awt.Component;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.image.VolatileImage;

public class VImageDemo extends Component {
	
    VolatileImage backBuffer = null;

    void createBackBuffer() {
	if (backBuffer != null) {
	    backBuffer.flush();
	    backBuffer = null;
	}
	backBuffer = createVolatileImage(getWidth(), getHeight());
    }

    public void paint(Graphics g) {
	if (backBuffer == null) {
	    createBackBuffer();
	}
	do {
	    // First, we validate the back buffer
	    int valCode = backBuffer.validate(getGraphicsConfiguration());
	    if (valCode == VolatileImage.IMAGE_RESTORED) {
		// This case is just here for illustration
		// purposes.  Since we are
		// recreating the contents of the back buffer
		// every time through this loop, we actually
		// do not need to do anything here to recreate
		// the contents.  If our VImage was an image that
		// we were going to be copying _from_, then we
		// would need to restore the contents at this point
	    } else if (valCode == VolatileImage.IMAGE_INCOMPATIBLE) {
		createBackBuffer();
	    }
	    // Now we've handled validation, get on with the rendering

	    //
	    // rendering to the back buffer:
	    Graphics gBB = backBuffer.getGraphics();
	    gBB.setColor(Color.white);
	    gBB.fillRect(0, 0, getWidth(), getHeight());
	    gBB.setColor(Color.red);
	    gBB.drawLine(0, 0, getWidth(), getHeight());

	    // copy from the back buffer to the screen
	    g.drawImage(backBuffer, 0, 0, this);

	    // Now we are done; or are we?  Check contentsLost() and loop as necessary
	} while (backBuffer.contentsLost());
    }

    public static void main(String args[]) {
	Frame f = new Frame();
	f.setSize(500, 500);
	f.add(new VImageDemo());
	f.setVisible(true);
    }
}

And now, on with the questions:

Q: So when do I use VolatileImage objects? Should I use them instead of BufferedImages, or managed images?
A: Ah yes, the plethora of image types and the confusion that quickly sets in. The quick answer is "You probably do not need to ever use a VolatileImage object directly in your application." This answer assumes that most applications:

• Are Swing apps, and therefore are already using a VolatileImage back buffer because Swing does it for them
• Do not need or want the extra complexity inherent in using these images that must be managed.
• Do not need every ounce of performance and runtime details available

The main reason for anyone to ever use a VolatileImage object is when you need a dynamic (frequently updated) image that you would really like to be accelerated if possible, including both rendering to the image and copying from the image. This is the basic usage model of a back buffer; you want it accelerated and you are rendering to the image frequently (at least once per screen update, by definition). This is why Swing uses a VolatileImage for its back buffer.

Using a BufferedImage or a managed image in this kind of scenario will instead give you software-rendering performance. Because of the volatile nature of accelerated memory (particularly on Windows), any image that is not explicitly managed for loss (which means any image that is not a VolatileImage) must reside primarly in system memory. That way, any rendering operation to that image is captured in that system memory version and cannot be destroyed through the loss of an accelerated version of the image. This implies that all rendering to the image must occur first through software rendering loops (since we cannot use hardware acceleration to render to a system memory image). Then, if it makes sense, we may copy that image into accelerated memory for future copies from that image. There are two main problems with this for a back-buffer scenario:

• Performance: for simple rendering, hardware acceleration will generally win out over software rendering performance. There are various reasons for this, including the speed of the graphics processors, the inherent parallelism of offloading operations to the graphics chip, etc. But in the end, just remember that's why they call it "accelerated". So if your back buffer resides in system memory (if you are not using a VolatileImage), then you must make do with software rendering performance.
• Performance: Although the render-to performance is important for the back buffer, sometimes the bigger win comes in the simple copy-to-screen operation that back buffers do on every frame. Ideally, this copy happens between an accelerated back buffer living in VRAM to the screen, also living in VRAM, and can happen at amazing speeds due to the memory bandwidth in the graphics system. But if your back buffer lives in system memory, this copy must happen over the system bus: reading from system memory (hopefully the on-chip cache, at least) and copying to VRAM down the PCI or AGP bus. Imagine trying to render 60 frames per second in an animation application, with a screen size of 1280x1024 and 32bpp screen depth; that's about 5 megs of data per frame * 60 frames = 300 MBytes of data moving across the bus, just to copy the buffer to the screen, not including anything else that needs to happen over the bus or through the CPU/cache/memory. "But what if the back buffer is a managed image?" Unfortunately, this is no better than an un-managed image; because you are rendering to the back buffer every frame, there is no benefit to our copying that buffer down to a VRAM cached version; we'd have to do it every frame (since the contents of the primary version of the image are being updated every frame), which is no better than simply copying it directly to the screen every frame.

Q: Couldn't the contenstLost() rendering loop become an infinite loop?
A: No. At least, not in theory. validate() will attempt to allocate an accelerated surface. Failing that, it will allocate the surface memory in system memory. So when you get either an IMAGE_OK or and IMAGE_RESTORED, you can guarantee that there is something there to render to (and from). And if you get an INCOMPATIBLE error, you must recreate the image completely, which again will result in at least something you can render to and from. So if nothing happens to that surface between that validate and the contentsLost(), then contentsLost() will return false and you can continue. The only situation I can foresee where you might get stuck in this loop more than one time around would be some weird system thrashing case where we are able to allocate VRAM, but then it's clobbered by the time we get around to the end of the loop, then we can get the memory again by the next validate, then it gets clobbered, and so on; this is pretty unlikely in reality, and if this is really happening on the system, there are probably more disturbing things to worry about than spinning in this rendering loop too many times.

Q: What's with the extra null-check condition in createBackBuffer()?
A: This is an optimization tweak for those interested. There are a couple of things going on here:

	backBuffer.flush();

The second item is this condition is even tweakier:

	backBuffer = null;

Let us say that the operation of assigning the new object to backBuffer causes the collector to run to try to get more memory. It can only get memory from objects that are already dead; but since the old backBuffer object is still alive until this reassignment is done, that object is not available for collection.

As I said, this extra step is somewhat useless here; since we already nuked the video memory taken up by this image by using the more definitive and direct approach of calling flush(). The only possible memory savings here is that Java heap memory taken up by the image data structure. This is not enough to worry about. But I brought it up because I have run into problems with this in other image contexts. For example, let's say (for some unknown and perhaps suspect reason) that our rendering loop creates a new BufferedImage every time through the loop for some intermediate calculation. Images can get pretty big, so this one object can be enough to swamp the young generation of the collector; but if we have to have 2 of these objects live simultaneously, then there's a fair chance that we're going to start chucking objects into the tenured generation and thus requiring Full (aka, "expensive") GC operations to get that memory back. Much better to get rid of the old one before asking for another one.

I don't want to get too deeply into GarbageCollection issues and implementations (and I think I've reached the limit of my knowledge on the subject in any case), but I did want to mention the cases above because as graphics and image users, we tend to run into unique garbage and memory situation, so it's worth understanding more about this stuff.

Example time: Let's say we have one VImage sprite and a VIimage backBuffer. We could write a buggy rendering loop like this:

		int valCode = backBuffer.validate(comp.getGraphicsConfig());
		if (valCode == VolatileImage.IMAGE_RESTORED) {
		    // backBuffer does not need to be restored,
		    // but sprite does - initContents() is a method
		    // that re-renders the contents into the sprite
		    initSpriteContents();
		} else if (valCode == VolatileImage.IMAGE_INCOMPATIBLE) {
		    // recreate all volatile images
		    createBackBuffer();
		    createSprite();
		    // also restore contents of sprite since we now have a
		    // completely new image
		    initSpriteContents();
		}
		// rendering to the back buffer:
		Graphics gBB = backBuffer.getGraphics();
		gBB.drawImage(sprite, 0, 0, this);
		
		// copy from the back buffer to the screen
		g.drawImage(backBuffer, 0, 0, this);
		
		// Now we are done; or are we?  Check contentsLost() and loop as necessary
	    } while (!backBuffer.contentsLost());

Here's the problem: images only get reinstated (put into a reasonable state to render to/from) when you call validate() on that image. So if you get a IMAGE_RESTORED value from backBuffer.validate(), then chances are pretty good that the sprite is also lost ... and is staying lost. The code above calls initSpriteContents() to re-render the contents of the sprite, but these operations will fail because the sprite is not in a state that will accept rendering.

The single contentsLost() call will probably work for now, but as I said above, if any images are every selectively punted that could change and the above code would be wrong under the mythical future implementation.

A more correct version of the above looks something like this:

		int valCodeBB = backBuffer.validate(comp.getGraphicsConfig());
		int valCodeSprite = sprite.validate(comp.getGraphicsConfig());
		if (valCodeSprite == VolatileImage.IMAGE_RESTORED) {
		    // note that we do not care about the back buffer
		    // for restoration since the buffer will be 
		    // re-rendered here anyway.
		    initSpriteContents();
		}
		if (valCodeBuffer == VolatileImage.IMAGE_INCOMPATIBLE) {
		    createBackBuffer();
		}
		if (valCodeSprite == VolatileImage.IMAGE_INCOMPATIBLE) {
		    createSprite();
		    // also restore contents of sprite since we now have a
		    // completely new image
		    initSpriteContents();
		}
		// rendering to the back buffer:
		Graphics gBB = backBuffer.getGraphics();
		gBB.drawImage(sprite, 0, 0, this);
		
		// copy from the back buffer to the screen
		g.drawImage(backBuffer, 0, 0, this);
		
		// Now we are done; or are we?  Check contentsLost() and loop as necessary
	    } while (!backBuffer.contentsLost() && !sprite.contentsLost());

VolatileImage: Now you See it, Now you Don't

Way back when we were first implementing the VolatileImage API, I had asked to come to the Swing staff meeting so that I could explain about the new VolatileImage API and why Swing should start using it.

I got up to the whiteboard, drew some pictures (probably horrible, but it's just a crutch for me when I explain things). I described this new image type, boiling it down to its basics:

1. It's going to be hardware-accelerated (depending on the runtime platform)
2. Rendering-to and Copying-from should be way faster than the current BufferedImage objects
3. It can go away at any time
4. Swing should use it for their back buffer

Then they asked me to go back to point #3; the whole disappearing-image thing. Apparently, they thought I was kidding.

It's true: VolatileImages can (and quite often will) go away and the API for VolatileImage was developed specifically to deal with that problem. All of the loss situations currently occur only on Windows.

You would think that we would actually be notified prior to these problems, so that we could prevent the loss, or backup the image, or something. But you'd be wrong; we only find out from Windows that there is a problem the next time we actually try to use the image. Surface loss situations arise from situations such as:

• Another app going into fullscreen mode
• The display mode changing on your screen (whether caused by your application or the user)
• TaskManager being run
• A screensaver kicking in
• The system going into or out of StandBy or Hibernate mode
• Your kid just yanked the power cord to the machine

Let's look at how you use the API to manage this situation. Then we'll step back and talk about other issues regarding these lossy beasts.

There are two parts to the API that need to be used in dealing with image loss:

	int valCode = validate(GraphicsConfiguration gc);
	boolean contentsLost();

validate() is used prior to using a VolatileImage. This tells us that you are about to use the image, so we need to make sure that it is in a state ready to be used. In most cases, all we do is check that things are fine and return IMAGE_OK. But in some situations, we need to do more:

• SurfaceLoss: One of the situations above that cause surface loss may have occurred since the last time you called validate(), in which case the underlying native surface memory is no longer there. In this case, validate() tells us to reallocate that memory or, failing that (there may not be enough VRAM, or some other failure might occur), allocate something in system memory that can also do the job. One of these is guaranteed to succeed and we will then return IMAGE_RESTORED from validate.
• Surface incompatibility: the reason that we take a GraphicsConfiguration argument to validate is that we need to know what you intend to do with the image before we can do the right thing to validate it. For example, you may have a dual-head system. Perhaps you created the VolatileImage on the primary graphics device. Later, you wish to copy that image to a Component on the secondary device. Surfaces are stored in graphics-device-specific memory, so we cannot copy an image from the VRAM on one device to another device. Or if we can, you probably don't want us to, because it would be a very slow operation. Instead, we ask you to tell us the target device for the operation and then we will tell you whether this is possible for the given image. If the GraphicsConfiguration that you pass in is compatible with the one that the image was created with, then we return IMAGE_OK, otherwise we return IMAGE_INCOMPATIBLE.

When you get a RESTORED error, you know that the image was in a funky state, but has now been reinstated and can be used. If you are about to write to the image and clobber the contents (as you would a back buffer, for example), this is not an error that you need to do anything about; the image memory exists and that's all you need to know. But if it's an image that you are copying from, or in some other way depending on the contents, then you need to restore those contents prior to depending on them.

For example, let's say you have an icon stored in a VolatileImage. At some point in your app, the image memory gets lost. The next time you go to copy from that image to the back buffer, you get the RESTORED code. Before doing the copy that you wanted to do, you need to recreate the icon using whatever means you need to do do that.

When you get an INCOMPATIBLE error, you know that your image is not in an appropriate state to do whatever operation you want to do with it. So, for example, if you are about to copy that image to a window with the GraphicsConfiguration that you passed in, that operation may not succeed. The only way to handle this error is to simply recreate the image on the new GraphicsConfiguration.

Now that you know what to do prior to using your image (call validate() and handle the return code appropriately), let's look at what you do afterwards. ("Afterwards? I'm done rendering, aren't I? How much do I have to do here? I thought Java was supposed to be easy, dangit!")

Notice that you had to call validate() prior to doing any rendering in case anything happened to your image since the last time you used it. But what happens if something happened to your image during your use of it? You might just want to know that some failure occurred. For example, let's say you are using a VolatileImage for a back buffer in some double-buffered application. You're in your rendering loop, you've called and handled the validate() call for the buffer, and you've done all the rendering to that buffer, and then copied the buffer to the screen. You're done, right?

Nope; what about if your image became lost during either the rendering to that buffer or the rendering from the buffer to the screen? Then chances are pretty good that either the copy to the screen simply didn't happen, or that some of the rendering either to or from that buffer is undefined. Do you want to just let it go and catch it later? Probably not, especially if this is some static application that only renders its contents based on changes in the app (versus some animation that's rendering one frame after another constantly); you don't want garbage hanging out on the screen or some required paint event just dropped on the floor. Instead, you want to make sure you get the right bits to the user as soon as possible.

This is where contentsLost() comes in; at the end of your rendering loop (or wherever you are doing rendering operations to and from your VolatileImage(s), you need to call contentsLost(). The boolean return value from this function will tell you whether the rendering operations from the iamge in question happened successfully, or whether anything went amiss during rendering. If the function returns false, everything is cool and you can just continue about your normal routine. if the function returns true, this means that the contents on that image are undefined and you need to:

• re-validate() that image
• redo whatever rendering you had just done to make sure it gets done correctly

So these are the two crucial pieces of the VolatileImage puzzle: you call validate() before you do anything with the image and contentsLost() after you are supposedly finished. And based on the return values from those functions, you do some other stuff to handle the errors.

We can put this into a simple loop which should probably look very similar in most situations. At first, I wrote some code that just showed the inner rendering loop. But to avoid confusion and allow easy copy/paste/compile/test, I've posted an entire (and functional) test app below. Sure it's boring (unless you really dig apps that draw single lines to the screen), but it shows the basic VolatileImage management we're after:

import java.awt.Color;
import java.awt.Component;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.image.VolatileImage;

public class VImageDemo extends Component {
	
    VolatileImage backBuffer = null;

    void createBackBuffer() {
	if (backBuffer != null) {
	    backBuffer.flush();
	    backBuffer = null;
	}
	backBuffer = createVolatileImage(getWidth(), getHeight());
    }

    public void paint(Graphics g) {
	if (backBuffer == null) {
	    createBackBuffer();
	}
	do {
	    // First, we validate the back buffer
	    int valCode = backBuffer.validate(getGraphicsConfiguration());
	    if (valCode == VolatileImage.IMAGE_RESTORED) {
		// This case is just here for illustration
		// purposes.  Since we are
		// recreating the contents of the back buffer
		// every time through this loop, we actually
		// do not need to do anything here to recreate
		// the contents.  If our VImage was an image that
		// we were going to be copying _from_, then we
		// would need to restore the contents at this point
	    } else if (valCode == VolatileImage.IMAGE_INCOMPATIBLE) {
		createBackBuffer();
	    }
	    // Now we've handled validation, get on with the rendering

	    //
	    // rendering to the back buffer:
	    Graphics gBB = backBuffer.getGraphics();
	    gBB.setColor(Color.white);
	    gBB.fillRect(0, 0, getWidth(), getHeight());
	    gBB.setColor(Color.red);
	    gBB.drawLine(0, 0, getWidth(), getHeight());

	    // copy from the back buffer to the screen
	    g.drawImage(backBuffer, 0, 0, this);

	    // Now we are done; or are we?  Check contentsLost() and loop as necessary
	} while (backBuffer.contentsLost());
    }

    public static void main(String args[]) {
	Frame f = new Frame();
	f.setSize(500, 500);
	f.add(new VImageDemo());
	f.setVisible(true);
    }
}

Stay tuned for my next blog; now that we've gone over the basics in how to use the VolatileImage API, we'll address some questions that you may have about using VolatileImages, or about this sample code in particular.

BufferedImage as Good as Butter, Part II

Let's dive briefly into some of the performance implications with BufferedImages (because I'm writing this and I tend to wind up in the performance arena no matter what I'm talking about).

There is currently only one image type that we guarantee acceleration on (if possible on the runtime platform): VolatileImage. When you create one of these images, we try to stash it in accelerated memory (such as VRAM on Windows, or an accelerated pixmap on Unix) and then perform rendering operations to and from that image using any available hardware acceleration.

VolatileImages work well for things like back buffers (ala the Swing back buffer, which is now a VolatileImage), where you obviously want to render to them frequently and copy from them as fast as possible. But for your average image, managing a VolatileImage can be tiresome (you have to make sure it's there before and after you use it), and you can't get all the flavors of images you want (currently only opaque volatiles exist).

But think about your average application: there's the back buffer and screen that you write to often, so you want those rendering-to operations to go really really fast. But then there's a bunch of other images like icons, sprites, whatever which you really only write to once or occasionally, but from which you would like to copy often.

This is where Managed Images come in (our new catch-phrase which, roughly translated, means "we will try our darnedest to accelerate this for you"). Here, you create an image however you need to, start working with it, and internally we will recognize that these copying operations can go much faster using an accelerated version, so we will just create that cached version for you. You don't need to manage the image and you don't need to know how these operations are happening; you just keep calling your rendering operations and let us take care of the pesky details.

Now, the fine print: as of 1.4.*, we hooked out only certain parts of the API to create these managed images. Specifically, you need to create an image either by calling one of the create*Image methods:

GraphicsConfiguration.createCompatibleImage(w, h)
GraphicsConfiguration.createCompatibleImage(w, h, transparency)
Component.createImage(w, h)

So in the current implementation of Java2D, the advice posted by ajsutton in response to Part I of this BufferedImage article is well-taken: if you want to take advantage of possible acceleration for your image, use a compatible image (or one of the other means above). Then we will attempt to accelerate this for you. Another benefit of using a compatible image is that you will get an image that is "compatible" (thus the name of the method above) with the display device you are rendering to, which saves pixel format conversion during the copy loops.

(A further caveat is that not all image types that you get from the above methods are acceleratable. For example, if you create an image with the flag Transparency.TRANSLUCENT then we do not currently accelerate that image and you end up going through software rendering loops regardless. Look for this to change as the library evolves and we try to accelerate more and more standard yet nifty features of the API).

Okay, so that's the state of things now: use BufferedImage for all of your condiment image needs, and if performance is particularly important to you, then use one of the variants mentioned above is the way to go. But what about the future?

Gosh, I'm glad you asked that. What a great question.

You may already be wondering, in reading the above explanations and caveats: "Why can't they just accelerate everything? Why are only portions of the API managed?" In fact, this is totally correct; there is nothing preventing this from happening (other than the most obvious of reasons: time to implement and lots of other stuff that we've been working on in the meantime). For example, let's say you have a 16-bit BufferedImage you created from scratch and you want to copy it to a 32-bit display. This means that we have to do a pixel-format conversion, so we can't cache the 16-bit version, right? But we can cache a new 32-bit version; we just copy the 16-bit version to our new 32-bit cached version, and then simply use the cached version thereafter.

Starting in jdk1.5 (currently in the oven, baking for a while, available at some unspecified (by me) date in the future), we will manage a much wider array of images. In fact, most of the images you can create or load will be managed for you. The code is in there, I've seen it working with my own eyes: BufferedImage objects running as fast as compatible images. It's pretty sweet...

That's all for now. I've glossed over many of the details of images and acceleration, but hopefully I've given a taste of how accelerated images work today and in the future. And hopefully you will be able to use this information to get the fastest and tastiest BufferedImage applications possible.

This work is licensed under a Creative Commons License.